The goal of this MHCoE activity is to develop new high-capacity lightweight metal hydride materials that both absorb and release hydrogen as close as possible to the DOE specific targets at low (less than 100°C) temperatures. Principal investigators included in this project are Ewa Rönnebro, Eric Majzoub, Mutlu Kartin and Vitilie Stavila.

Our synthesis methods involve high-energy milling using a SPEX 8000 instrument and an in-house high-pressure facility for heat treatment of powder precursors at below 500°C and 2000bar (140MPa) H₂-pressure. We are using a commercial autoclave which can contain six samples, enabling an effective way to screen for new materials.

For analysis we have powder X-ray diffraction (XRD), Thermogravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC), Raman Spectroscopy, FTIR, NMR, SEM, TEM, and PCT-instruments available at Sandia. A picture of a PCT instrument is shown in Figure 1 below.

Figure 1. PCT instrument for characterization of hydride material’s absorption and desorption properties.

We successfully prepared calcium borohydride (Ca(BH₄)₂) by using a previously untried solid-state reaction path that was predicted by theoretical calculations to result in a reaction enthalpy of ~53 kJmol^-1 and 9.6 wt% (materials basis) reversible storage:

Further, additives are crucial to increase the formation of Ca(BH₄)₂. The decomposition products of Ca(BH₄)₂ were first predicted by theory to be CaB₆ and CaH₂.

Figure 2 shows the resulting data from TGA (red) and DSC (black) analyses of Ca(BH₄)₂. The data show a reversible low-temperature to high-temperature endothermic phase transition at 140°C and another endothermic phase transition at 350-390°C associated with hydrogen release. We are currently working on lowering the hydrogen desorpsion temperature.

Figure 2. DSC data (black) and TGA data (red) for Ca(BH₄)₂ showing two endothermic phase transformations at on set temperatures 140ºC and at 350ºC, and release of hydrogen at 350ºC

We have successfully explored the features of Ca(BH₄)₂ by different analysis techniques: DSC, TGA and Raman spectroscopy at Sandia, in-situ powder XRD with U. Nevada-Reno (Prof. Chandra), neutron vibrational spectroscopy with NIST (Dr. Udovic), NMR with JPL and Caltech (Dr. Bowman and Dr. Hwang) and in-situ synchrotron analysis with GE (Dr. Zhao et al). We observed partial reversibility below 100 bar H_2-pressure, and are currently identifying all intermediate phases occurring during desorption/absorption from Ca(BH₄)₂.

Inspired by the work on calcium borohydride, we have in collaboration with our MHCoE partner Prof. Jensen at the University of Hawaii, undertaken high-pressure experiments at our high-pressure station to prepare a series of high-capacity alkali and alkali earth transition metal borohydrides. The synthesis routes involve mixing of transition borides with alkaline metal hydrides by high-energy mechanical milling and thereafter, heat treating at the high-pressure station.

We are also exploring bialkali borohydrides to find new high-capacity hydrogen storage materials by solid-state reactions predicted by theory. As an example, a new bialkali borohydride, NaK(BH₄)₂, was predicted to be metastable and was recently synthesized via a ball milling technique. The material indeed proved to be metastable and decomposed within a day. We are continuing exploring other potential bialkali borohydrides.